Precise distributed borehole temperature measurements are important for completion and production monitoring. For example, different borehole temperatures for unconventional wells reflect different fracture geometry and completion results for hydraulic fracturing operations. Borehole temperature anomalies are the indicator of leakage or well integrity problems. Temperature variations during production can be used to estimate production allocation of different well sections. Therefore, accurate borehole temperature information can provide better production estimation and better planning and execution to safely optimize production.
Many methods have been proposed to measure borehole temperatures. Placing a number of temperature sensors at different depths of a borehole is a simple way to accomplish the task, but this method cannot provide continuous data along the borehole, especially when the wells are drilled several miles in different directions.
Distributed Temperature Sensing (DTS) is a way of measuring temperature in a continuous manner. DTS systems are optoelectronic devices that measure temperature by means of optical fibers functioning as linear sensors. Temperatures are recorded along the optical sensor cable, thus not at discrete widely separated points, but as a continuous profile. Temperature determination is achieved over great distances. Typically the DTS systems can locate the temperature to a spatial resolution of 1 m with accuracy to within ±1° C. at a resolution of 0.01° C. Measurement distances of greater than 30 km can be monitored and some specialized systems can provide even tighter spatial resolutions.
Physical measurement dimensions, such as temperature or pressure and tensile forces, can affect glass fibers and locally change the characteristics of light transmission in the fiber. By damping the light into the quartz glass fibers and measuring the arrival time of the backscattered light, the location of an external physical effect can be determined so that the optical fiber can be employed as a linear sensor. Optical fibers are made from doped quartz glass. Quartz glass is a form of silicon dioxide (SiO2) with amorphous solid structure. The light scattered back from the fiber optic therefore contains three different spectral shares:                the Rayleigh scattering with the wavelength of the laser source used,        the Stokes line components from photons shifted to longer wavelength (lower frequency), and        the anti-Stokes line components with photons shifted to shorter wavelength (higher frequency) than the Rayleigh scattering.        
One kind of the light scattering, also known as Raman scattering, occurs in the optical fiber. Unlike incident light, this scattered light undergoes a spectral shift by an amount equivalent to the resonance frequency of the lattice oscillation. Thermal effects induce lattice oscillations within the solid. When light falls onto these thermally excited molecular oscillations, an interaction occurs between the light particles (photons) and the oscillation of the molecule. The intensity of the so-called anti-Stokes band is temperature-dependent, while the so-called Stokes band is practically independent of temperature. The local temperature of the optical fiber is derived from the ratio of the anti-Stokes and Stokes light intensities.
There are two basic principles of measurement for distributed sensing technology, Optical Time Domain Reflectometry (OTDR) and Optical Frequency Domain Reflectometry (OFDR). For Distributed Temperature Sensing often a Code Correlation (CC) technology is employed, which carries elements from both principles.
OTDR was developed more than 20 years ago and has become the industry standard for telecom loss measurements and detects Rayleigh backscattering signals, which dominate the weaker Raman signals. The principle for OTDR is quite simple and is very similar to the time of flight measurement used for radar. Essentially a narrow laser pulse generated either by semiconductor or other solid state laser is sent into the fiber and the backscattered light is analyzed. From the time it takes the backscattered light to return to the detection unit it is possible to locate the location of the temperature event.
Alternative DTS evaluation units deploy the method of Optical Frequency Domain Reflectometry (OFDR). The OFDR system provides information on the local characteristic only when the backscatter signal detected during the entire measurement time is measured as a function of frequency in a complex fashion, and then subjected to Fourier transformation. The essential principles of OFDR technology are the quasi continuous wave mode employed by the laser and the narrow-band detection of the optical backscatter signal. This is offset by the technically difficult measurement of the Raman scatter light and rather complex signal processing, due to the fast Fourier transform (FFT) calculation with higher linearity requirements for the electronic components.
Code Correlation DTS sends on/off sequences of limited length into the fiber. The codes are chosen to have suitable properties, e.g. Binary Golay code. In contrast to OTDR technology, the optical energy is spread over a code rather than packed into a single pulse. Thus a light source with lower peak power compared to OTDR technology can be used, e.g. long life compact semiconductor lasers. The detected backscatter needs to be transformed—similar to OFDR technology—back into a spatial profile, e.g. by cross-correlation. In contrast to OFDR technology, the emission is finite (for example 128 bit) which avoids that weak scattered signals from afar are superposed by strong scattered signals from a short distance, thus improving the shot noise and the signal-to-noise ratio.
For example, U.S. Pat. No. 8,630,816 (Park et al.) provides a high spatial resolution DTS sensor having a resolution of 25 mm or less, but does not address the temperature resolution issue. U.S. Pat. No. 8,646,968 (MacDougall et al.) provides a DTS sensor in hydrogen environment having an accuracy of ±1.7° C. U.S. Pat. No. 8,930,143 (Sierra et al.) provides a method of enhancing DTS resolution by using a thermal tracer (a temperature anomaly) in repeated acquisition and multiple functions, but no actual measurement data was provided.
DAS is an acoustic detection technology that has recently been applied in production and geophysical settings. Downhole DAS is a fiber-optic distributed sensing technology that can provide key diagnostic insights during hydraulic fracturing operations.
DAS is the measure of Rayleigh scatter distributed along the fiber optic cable. A coherent laser pulse is sent along the optic fiber, and scattering sites within the fiber cause the fiber to act as a distributed interferometer with a gauge length approximately equal to the pulse length. The intensity of the reflected light is measured as a function of time after transmission of the laser pulse. When the pulse has had time to travel the full length of the fiber and back, the next laser pulse can be sent along the fiber. Changes in the reflected intensity of successive pulses from the same region of fiber are caused by changes in the optical path length of that section of fiber. This type of system is very sensitive to both strain and temperature variations of the fiber and measurements can be made almost simultaneously at all sections of the fiber.
Raw DAS data are usually in the form of optical phase, with a range from −pi to +pi. The optical phase is defined by the interference pattern of the back-scattered laser energy at two locations separated by a certain length (gauge length) along the fiber. The phase varies linearly with a small length change between these two locations, which can be interpreted as axial strain change of the fiber in between. Depending on the vender, the measured optical phase is sometimes differentiated in time before it is stored. In this case, the DAS data can be considered as linear scaled fiber strain rates.
In practice, fiber-optic cables can be installed in vertical and horizontal wells, which can be treatment wells, injector wells, production wells, or monitor wells. Within the cable, there are often both single mode fibers for DAS and multi-mode fibers for DTS. Multiple fibers within one cable can offer redundancy and the ability to interrogate with different instrumentation simultaneously.
The current solution using DTS can only provide the temperature measurement with an accuracy around 0.1° F. On the other hand, the low-frequency component of Distributed Acoustic Sensing (DAS) as described in U.S. Provisional Application Ser. No. 62/305,758 is very sensitive to the temperature variation, with an accuracy up to 10−5° F./s, but DAS cannot measure absolute temperature, thus making the temperature change relative in the downhole context.
US20150146759 (by Johnston) describes a system combining of DAS and DTS temperature measuring devices, but failed to illustrate the fitting models or relationship between DAS data and DTS data to obtain a meaningful temperature reading. Although US20150146759 can measure the temperature change for a certain time interval, it provides no method to numerically combine the DAS and DTS data to get continuous and precise absolute temperature measurement.
Therefore, there is a need for an efficient way of measuring borehole temperature that is accurate and responsive so as to calculate the optimal completion design and production optimization.